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Abstract

We present a direct-method solution toward the general problem of plasmonic wavefront manipulation and shaping to realize pre-designated functionalities based on the surface-wave holography (SWH) method. We demonstrate theoretically and experimentally the design and fabrication of holographic plasmonic lenses over surface plasmons with complex wavefront profiles. We show that visible light at 632.8 nm transmitting through a high-aspect-ratio slit or a micro-rectangle hole in a silver film can be focused to a preset three-dimensional point spot in free space via appropriately manipulating the interaction of excited surface plasmons with the nanoscale groove pattern of the holographic lens. The experiment results of scanning near-field optical microscopy for measuring the three-dimensional optical field distribution agree well both with designs and with numerical simulations, and this strongly supports the effectiveness and efficiency of the SWH method in the design of plasmonic devices that can fulfill manipulation and transformation of complicated-profile surface plasmons.

Figures (6)

The three steps of the surface-wave-holography method. (a) The wavefront of the objective wave U0. Place a point source with x polarization at (0, 0, 7) μm, calculate its propagation and store the field distribution at z = 0 μm as U0. (b) The wavefront of the reference wave Ur. An x-polarized incident light is shined to an aperture in a 240-nm-thick silver film. The field distribution immediately above the surface of the silver film is stored as Ur. (c) The designed sample. Grooves are fabricated at positions where the phase of U0Ur equals 2mπ.

(a) The SEM photo of the slit sample. The measured size of the slit is 11.10 × 0.15 μm2 (the value in design is 11 × 0.12 μm2). The measured x scale of the structure is 12.18 ± 0.02 μm, and the y scale is 12.14 ± 0.02 μm (both are 12 μm in design). Shown in the bottom is a scale bar of 2 μm. (b) The simulated field distribution of the surface waves excited by an 11 × 0.12 μm slit.Note that the silver surface in (b) is not patterned with grooves . (c) The simulated field distributions at z = 7 μm above the patterned slit sample. The calculated intensities in (b) and (c) are normalized to the incident wave intensity.

The SNOM measured field distributions at different heights above the slit sample. Heights are from z = 0.5 μm to z = 10 μm, as noted. The scan areas of all experimental pictures shown here are 20 × 20 μm2 (note that the milled structure is approximately 12 × 12 μm2). The absolute measured intensity (in arbitrary units) of a photomultiplier is shown here.

The field evolutions in the z direction. (a) The simulated and (b) the experimental field distributions in the x = 0 yz plane. (c) The simulated and (d) the experimental field distributions in the y = 0 xz plane. In the simulated figures (a) and (c), the calculated intensities are normalized to the incident waves, while the intensities shown in the experimental figures (b) and (d) are normalized by the divided-by-maximum method.

(a)The SEM photo of the fabricated micro-hole sample. A scale bar of 2 μm is shown in the bottom. The area with structure are (12.17 ± 0.04) × (12.17 ± 0.02) μm2, whose value in design is 12 × 12 μm2. (b) The simulated field distribution of the surface waves excited by a 3 × 0.12 μm slit.Note that the silver surface in (b) is not patterned with grooves. (c) The field distributions at z = 7 μm above the micro-hole sample. The caculated intensities are normalized to the incident wave.

The SNOM measured field distributions at different heights above the slit sample. Heights are from z = 0.5 μm to z = 10 μm, as noted. The scan areas of all experimental pictures shown here are 20 × 20 μm2. The absolute measured intensity (in arbitrary units) of a photomultiplier is shown here.